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7.2.1 Fibre effects

Although reinforcing fibres like carbon, boron, and other ceramic reinforcements are insensitive to the effects of moisture, others, particularly glass and aramid fibres like Kevlar-49, are affected by moisture even at low exposure levels. Glass and Kevlar-49 fibres are known to have mechanical properties that are dependent on time and temperature, giving rise to the familiar ‘static fatigue’ effect illustrated in Figure 7.2 (Aveston et al, 1980) which is characterized by the fact that the time to failure under a constant load depends on the load level.

In the case of glass, this is a form of environmental stress cracking, or stress-corrosion, resulting from the leaching out of the network modifier, NaO2, from the glass structure. The resulting alkaline environment then attacks the normally-stable SiO2 network and reduces the strength of the glass fibre. Stress-corrosion essentially results from the conjoint effects of the corrosive environment and the applied stress. The effect occurs more rapidly in acid or alkaline environments than in water alone, but even in pure water the rate of loss of strength is high and the extent of the weakening is substantial. The loss of strength of ordinary E-glass fibres in the

alkaline environment of a damp concrete matrix seriously limited the effective use of glass-fibre- reinforced cement products (GRC) in the building industry. This problem led to development at the UK Building Research Establishment, and subsequent exploitation by Pilkington, of a zirconia-containing glass known as ‘Cem-FIL’, which is more alkali- resistant than other glass compositions (Majumdar, 1970). Results on the stress-corrosion of this Cem- FIL glass fibre in aqueous alkaline environments (Proctor and Yale, 1980) suggested that the material offered a far-from-perfect solution, but it has been shown that under the more realistic conditions of service exposure of thin-walled Cem-FIL-reinforced cement pipe the practical life of such materials is greater than the laboratory results of Proctor and Yale suggested (ARC, 1979).

-1 0 1 2 3 4 5 6 7 0 2 4 6 8 glass Kevlar-49 carbon Load, k g

Log(t) (failure time, t min.) Figure 7.2. Time-dependent failure of carbon, glass and

As the results in Figure 7.2 show, the stress- rupture curve for carbon filaments is horizontal, the fibre therefore being completely insensitive to the environment, while Kevlar-49 shows a time- dependent response somewhere between that of carbon (nil) and that of glass (high). The aramid fibre, like glass, is therefore also environmentally sensitive although the stress-corrosion mechanism is different. The yarn may absorb some 6% by weight of water in about 30 hours at a relative humidity of 96% (Smith, 1979). This moisture absorption is accompanied by dimensional changes which are of importance for the manufacturers of composites. Any absorbed moisture must be removed from the yarn before prepregging or other laminating procedures since drying out after cure may lead to fibre/matrix decohesion and consequent loss of composite properties. Moisture absorption at

ambient temperatures does not impair the short-term strength of the fibre, but Smith (1979) has shown that high-temperature wet-ageing seriously reduces its strength.

7.2.2 Resin effects

The open molecular structure of glassy thermoplastic and thermoset polymers permits relatively rapid diffusion of moisture. Depending on the nature of the polymer, considerable quantities of moisture may be absorbed into the material, and both the rate of absorption and the saturation absorption level will depend on the conditions of exposure (ie. temperature and relative humidity). This is illustrated in Figure 7.3 by some results for an experimental epoxy resin manufactured by Elf-Aquitaine.

Moisture ingress in polymers usually follows Fickian kinetics, the amount of moisture absorbed being proportional to √(time) in the early stages of exposure, as predicted by the one-dimensional Fickian model of Shen and Springer (1981), for example. A condition for the applicability of Fick’s Law is that the diffusion is concentration-

independent, but the diffusion behaviour of many polymers, both glassy and cross-linked, under particular conditions, may not be adequately described by a law which predicates fixed boundary conditions, especially when the movement of the diffusing species is linked with molecular relaxations or morphological effects in the polymer. Deviations from Fickian behaviour are then observed which may be associated with structural changes, resin degradation, or even simple relaxation in response to the diffusing species. Frequently, the early response is Fickian and this may be followed by a quasi- equilibrium stage with a much slower approach to final true equilibrium (dual-mode sorption). True Fickian and dual-mode sorption effects are illustrated in Figure 7.4 by some results obtained at two different humidities for Ciba-Geigy 914 epoxide resin (Fernando, 1986).

The equilibrium moisture content depends on the chemical and physical nature of the polymer. The absorption of water strongly reduces the mechanical properties of hydrogen-bonded polymers like conventional polyamides, but in polyester and epoxide resins it acts as a diluent, increasing the free volume and facilitating molecular motion under stress. This plasticizing effect reduces the stiffness of

0 1000 2000 3000 4000 0 1 2 3 4 5 6 water at 70°C water at 50°C water at 20°C 65% RH at 23°C W e ight gai n , %

Exposure time, hours

Figure 7.3. Moisture absorption in Elf 37SA epoxy resin during exposure to wet or moist environments at various

temperatures (Charrière, 1985). 0 20 40 60 80 100 0 2 4 6 8 10 slope = 4M_mh-1_√(D/π) √(t), (min)½ 100% RH 65% RH dual-mode absorption M_m Fickian Moi s tu re abs o rp ti on, %

Figure 7.4. Moisture absorption curves for 914 epoxy resin at 23°C and two different levels of relative humidity (RH)

the resin, lowers the glass-transition temperature, Tg, and increases the magnitude of time-dependent effects like stress relaxation. The reduction in Tg by moisture absorption may be by as much as 100°C in some cases, as Figure 7.5 from the work of de Iasi and Whiteside (1978) shows. Crystalline thermoplasts are much more resistant to moisture penetration than epoxide resins. The semi-crystalline high-performance polymer poly(ether ether ketone) (PEEK), for example, appears to be largely immune from environmental effects. Mensitieri et al (1996) report only a 2°C depression of Tg in samples equilibrated in water at 60°C. Although PEEK contains oxygen atoms that may potentially form hydrogen bonds, it is not moisture sensitive because these atoms are sterically shielded by the aromatic rings.

The wetting and drying of a resin are accompanied by dimensional changes which generate residual stresses in a resin containing rigid reinforcement. Solvent attack on resins can cause both physical and chemical effects, the latter usually being either hydrolysis (of the ester linkages in a polyester, for example) or oxidation, although these processes are slow at ambient temperatures. Chemical attack naturally reduces the mechanical properties of the common resins.

7.2.3 Composite effects

In a normal composite material, moisture will enter by diffusion through the bulk resin, by capillary flow through pores in the resin, and also along the fibre/resin interface. Fickian kinetics again often appear to apply, but deviations from Fickian behaviour are frequently observed, such as where accelerated moisture pick-up occurs as a result of the presence of micro-voids or cracks in the resin. The moisture diffusion in an anisotropic composite lamina is of course also likely to be anisotropic, the rate of diffusion being more rapid in the direction of fibre alignment because of the continuity of the resin diffusion path. The kinetics of moisture uptake are there also markedly influenced by the lay-up of a laminate (Collings & Stone, 1985).

For the case of Fickian diffusion (ie. assuming that moisture transport is through the resin only) the diffusion coefficients for a single unidirectional lamina can be approximated by:

D11 = (1 – Vf)Dr... 7.1)

D22 = [1 – √(Vf/π)]Dr... 7.2) for diffusion parallel and perpendicular to the fibres, respectively, D being the diffusion coefficient for the matrix resin (Dr » Df). The diffusivity at any angle, θ, to the fibre direction is then

2 2

11 22

D_θ =D cos θ +D sin θ... 7.3) If the response of a single lamina is known (or can be calculated from models such as those of Springer) the overall moisture absorption behaviour of a multi-ply laminate can also therefore be calculated. Computer programs for this purpose have been developed (Curtis, 1981).

Bueche and Kelly (1960) developed a theory for the prediction of the Tg of plasticized resin composites which gives:

r f gr f f gf g r f f f (1 V )T V T T (1 V ) V α − + α = α − + α ... 7.4) 0 2 4 6 8 10 0 50 100 150 200 250

results for five common epoxy resins

G la s s t ra n s it io n t e m p er at ur e, Tg , ° C Moisture content, %

Figure 7.5. Effect of moisture content on the glass transition temperature of common epoxide resins

where α is a thermal expansion coefficient and V_f is the fibre (or filler) volume fraction, the subscripts r and f referring to the resin and fibre (or filler) components. The theory predicts the trend in Tg reasonably well. The nature of the fibre can also exert an influence on the process of moisture uptake, as illustrated by the graphs in Figure 7.6. The materials are of the same 0/90 lay- up and contain the same epoxide matrix resin, and it can be seen that at both ambient temperature in moist air and in boiling water there are only slight differences between the absorption in composites containing carbon and glass fibres, whereas the absorption in the aramid-fibre composite is substantially greater as a consequence of the hydrophilic nature of the aromatic polyamide.

Both reversible and irreversible mechanisms of degradation may occur as a result of moisture

ingress. For example, if water travels down the interface or is able to accumulate at the interface after diffusion through the bulk resin it may cause hydrolytic breakdown of any chemical bonding between the fibre and the resin and bonds may be disrupted by the swelling of the resin. Either of these mechanisms of bond failure will impair the efficiency of stress transfer between matrix and reinforcement. The strength and stiffness of the composite will then be recovered on drying out only if new chemical bonds can be re-established. If the bonding is simply mechanical, however, and the absorption and desorption of water are purely physical processes related to the resin or the interfacial bond (ie. supposing the fibre strength to be also unaffected), the composite would be expected to recover its properties on drying out. Thus, while the tensile strengths of carbon and Kevlar laminates are little affected by hygrothermal treatments, even as severe as boiling in water, a GRP laminate may lose some 50% of its strength. However, much of this damage is apparently reversible since even boiled GRP laminates may regain all or most of their original strengths on re-drying (Dickson et al, 1984). This suggests that in carbon and Kevlar composites there is only a physical bond between fibres and matrix, and that this is not affected by hot/wet ageing, whereas in the GRP there is a chemical bond (presumably resulting from the presence of a silane coupling agent on the fibres) which is destroyed by the hygrothermal treatment but apparently re-established on removal of the moisture. The marked change in the fracture surface of a GRP laminate after exposure to boiling water is shown in Figure 7.7: a failure surface of this kind, which is in marked contrast to the normal bushy fractures shown in Figure 5.7 of chapter 5, is inevitably associated with brittle behaviour.

The reversibility of this moisture effect, and the fact that the laminate behaviour goes from brittle in the dry state to ductile when wet and back to brittle on re-drying, supports the general belief in the plasticizing effect of moisture on the resin. It should be pointed out, however, that other experimenters, working with other resins and different combinations of environmental and stress conditions, have observed irreversible damage, such as resin cracking and permanent bond breakdown, with the inevitable irreversible degradation of composite properties. The chemical stability and resistance to moisture penetration of the matrix polymer, which depend sensitively on chemical and molecular structure and state of cure, will usually determine the extent of both reversible and irreversible composite damage, although resin void-content and composite fibre-content will also exert an effect. It is generally

0 200 400 600 800 1000 0 1 2 3 4 5 6 GRP; 65%, 23°C CFRP; 65%, 23°C CFRP & GRP; H2O, 100°C KFRP; 65% RH, 23°C KFRP; water, 100°C W e ig ht g a in, % ( fr o m dry state )

Exposure time, days

Figure 7.6. Absorption of water during exposure of composite laminates at ambient temperature (65% relative humidity) and at 100°C (in water). The laminates are 0/90 laminates of HTS carbon, E-glass

and Kevlar-49 in Code-69 epoxy resin.

Figure 7.7. Tensile fracture surface of an E- glass/epoxy GRP laminate after boiling in water

recognized that laminates based on polyester resins are less moisture resistant than epoxy- based composites, and that phenolic laminates are more water-resistant than either of these. But within each group of resins there is in fact a wide range of individual types and compositions, from which the designer may select according to his requirements, with a correspondingly wide range of chemical stability and moisture resistance. In the polyester range, for example, there is increasing chemical resistance through the sequence orthophthalic, isophthalic, neopentyl-glycol, and bisphenol polyester resins. Much of the research on matrix resins has as a general goal the improvement of the polymer tolerance to hot/wet conditions. One of the difficulties here is that improvements in hygrothermal tolerance can usually only be

achieved at the cost of a raised processing temperature. In Figure 7.8, for example, we see that the saturation uptake of water into a carbon-fibre composite based on Ciba-Geigy 913 resin is twice that in a similar laminate made with the same company’s 914, and as we saw in chapter 1, the former is processed at 120°C while the latter requires a processing temperature of 170°C.